Once they get back to their Spaceships (Red and Blue Plastic Covers) the Magician proceeds to explain that the front of the Spaceships are the same as the backs of both.

He then turns them around and when the Aliens come out they have switched places...

After a little fun with the kids, who are accusing the Magician of just turning them around, and saying the Aliens are Red on one side and Blue on the other, the Magician finally turns them around and reveals that they are actually Green and Yellow on the other side.